The present invention relates to a process for the continuous preparation of an acrylic polymer, and more particularly to an improved process for continuously preparing an acrylic polymer by photopolymerization of a monomer on a moving support.
It is known to prepare acrylic polymers by irradiating light energy to acrylic monomers. It is also known to continuously conduct the polymerization by irradiation to monomers in the form of a thin layer on a moving support such as a belt. In this technique, water-soluble acrylic polymers have been prepared usually by continuously feeding an aqueous monomer solution in the form of a thin layer onto a moving support, irradiating a light energy to the thin layer and continuously taking the produced sheet-like polymer gel off the support.
In general, in the case of polymerizing acrylic monomers by irradiation of light energy using, for example, a ultraviolet lamp, polymers of a good quality cannot be prepared stably unless thorough attention is paid to the following points, namely:
(1) that light energy irradiation conditions are always maintained constant during the polymerization period;
(2) that when the polymerization is conducted in the form of a thin layer on a moving support, the concentration of monomer in a monomer solution does not noticeably change resulting from vaporization of the solvent from the monomer solution (e.g. water when the monomer solution is an aqueous solution) by heat of reaction;
(3) that incorporation of oxygen into the monomer solution which has an adverse influence on the polymerization is completely eliminated during the polymerization period; and
(4) that the thickness of the monomer solution and the produced polymer gel on a moving support are controlled constant in the polymerization stage.
In general, from the viewpoint of productivity, it is desirable that the concentration of monomer in a monomer solution is high or the layer of the monomer solution on a moving support is relatively thick. However, in those cases, the quantity of heat generated by the polymerization remarkably increases, and even if the layer is cooled, for example, by cooling the reverse side of the moving support with cold water, complete removal of the heat of polymerization reaction is difficult. Consequently, the upper part of an aqueous monomer solution, which turns to a gel in a short time after irradiation of light energy, becomes in a bumping state. Also, the solvent vaporizes and the monomer concentration remarkably increases at the surface region of the gel. As a result, the concentration of monomer in the gel becomes ununiform, resulting in variation in rate of polymerization and degree of polymerization. Thus, the products having a stable quality cannot be obtained.
Further, the bumping from the surface of the gel results in scattering of the monomer itself. For example, when the monomer solution is irradiated directly with a ultraviolet lamp, the monomer sticks to the surface of lamp, and when a transparent partition plate such as glass or synthetic resin plate is provided between the ultraviolet lamp and the layer of the monomer solution, the monomer sticks to the partition plate. The monomer which has stuck to the surfaces of the lamp or partition plate, converts to a polymer on their surfaces. Thus, the effective output of the ultraviolet lamp, namely illuminance (W/m.sup.2), remarkably decreases. Moreover, in the case of long term operation, this decrease progresses with the lapse of time and, therefore, it becomes impossible to carry out the polymerization at a constant illuminance. The change of illuminance with the lapse of time exerts an influence on the rate of polymerization and the degree of polymerization. Thus, it is very difficult to assure a stable quality over a long term. Accordingly, frequent cleaning and washing are required, thus the productivity is remarkably lowered. For such reasons, an improvement has been damanded in this respect.
It is desirable to completely remove oxygen in the polymerization step, namely dissolved oxygen included in a monomer solution and oxygen included in a gas atmosphere in a polymerization apparatus. Removal of oxygen has been usually conducted. In a conventional process where the polymerization is carried out in the form of a thin layer on a moving support, removal of oxygen in the gas atmosphere, in other words, prevention of incorporation of oxygen into the monomer solution or contact of oxygen with the surface of the monomer solution, is conducted, for instance, (1) by installing the moving support in an oxygen-free room, (2) by arranging a gas tight chamber over the moving support and introducing an inert gas such as nitrogen or carbon dioxide to keep an oxygen-free state, or (3) by covering the surface of the monomer with a light-permeable film immediately after feeding the monomer solution onto the moving support.
The above-mentioned method (1) is the most preferable, but requires a high equipment cost because the machine operation in an oxygen-free room must be done by remote control. Also, even if operable, the oxygen-free state in the room must be released once when machine troubles occur or when abnormality in polymerization occurs. Frequent occurrence of such troubles results in marked decrease of productivity and waste of inert gas.
The arrangement of gas tight chamber over a moving support according to the above method (2) has the disadvantage of accompanying a waste of inert gas, since the gas tight chamber itself is not movable and, therefore, a clearance must be provided between the chamber and the moving support and the inert gas inside the chamber leaks more or less through the clearance. A movable support such as belt used in an industrial production is for example as long as 50 m. If a gas tight chamber of 50 m in length, 1 m in width and 0.3 m in height is arranged over the movable belt, the volume of the chamber is 15 m.sup.3 and an appreciable amount of inert gas is required for replacement of air inside the chamber. Moreover, in order to keep the pressurized state, it is necessary to always supply the inert gas to the chamber, for example, in an amount of 10 to 20 m.sup.3 /hour. The use of inert gas does not contribute to the yield of products and accordingly should be minimized from the economical point of view.
The above-mentioned method (3) dissolves the problems encountered by the methods (1) and (2), but has the disadvantage that since the covering film is in contact with the layer of the monomer solution before initiation of the polymerization, variation in thickness of the polymer gel owing to wrinkle occuring at the supply of the film or wrinkle resulting from shrinkage of the film caused by the heat of reaction. Ununiform thickness of the layer results in variation in polymerization rate. Further, since an inert gas is not introduced, oxygen enters to hinder the polymerization when the film is damaged or broken, thus no homogeneous polymer gel is obtained. Also, the continuous operation is interrupted by breakage of the film.
Another problem encountered by the process wherein a monomer solution is polymerized in the form of a thin layer on an moving support is that the peelability between the produced polymer gel and the moving support is poor when the support is made of a metal and when the produced polymer gel is very sticky, thus continuous operation is difficult.
Also, Japanese Patent Publication Kokai No. 60-149613 and No. 60-149612 disclose a process for preparing a water-soluble vinyl polymer by photopolymerizing a thin layer of an aqueous solution of a water-soluble vinyl monomer on a moving support, wherein a water-insoluble material such as an alkylene oxide adduct, a solid paraffin or a polydimethylsiloxane silicone oil is applied to the surface of the thin layer in the form of gel for the purpose of preventing vaporization of water and contact with oxygen in the gas atmosphere. According to this process, the obtained sheet-like polymer gel is ununiform in thickness and the both surfaces of the sheet-like gel is uneven.
It is known to provide a coating of polytetrafluoroethylene, polyester resin or the like on the surface of a movable support in order to improve the peelability of the produced sheet-like polymer from the moving support. However, the coating wears away during repeated use and the its peeling effect decreases. Also, the coating scratches easily. The monomer solution enters into the scratches and polymerizes therein, and when the produced polymer is forced to peel off from the moving support, the polymer getting into the scratches like anchor remains in the scratches and thereafter the peeling becomes difficult at these portions. Like this, when the coating scratches or exfoliates, the repair is required and the stop of the operation is unavoidable. In an industrial production of polymers, repair or recoating of a long-sized support results in a large loss of time and cost.
It is an object of the present invention to eliminate the above-mentioned defects of conventional processes.
This and other objects of the present invention will become apparent from the description hereinafter.